Use imports inside interface definition

I'm having a strange problem with typescript interfaces. Because I'm using mongoose models I need to define one, but for some reason it's not recognising things that I have explicitly imported. This part works fine:

export interface ITrip extends mongoose.Document {
//
}

export var TripSchema = new mongoose.Schema({
//
});

export var Trip = mongoose.model<ITrip>('Trip', TripSchema);

Now, I'm defining another interface, that has an array of Trip. I need this for subdocuments.